1. Field of the Invention
The present invention relates generally to a Gauss type lens system and more particularly to an improved Gauss type lens system of relatively high aperture ratio and long back focal distance having a five group, seven lens element design of a relatively compact configuration.
2. Description of the Prior Art
As is known in the prior art, a Gauss type lens system of high aperture ratio and long back focal distance does not readily lend itself to the correction of spherical aberration, coma and astigmatism. Complicating the solution of these problems is the commercial demand to provide a shorter overall length to provide a general overall compactness. Any attempt, however, to reduce the overall length of such a Gauss type lens system necessarily results in a substantial increase in coma flare at intermediate field angles, thus impairing the image performance of the lens system.
Optical designers are well aware of the five group, seven lens element Gauss type lens system format. In this regard, it has been a conventional practice as can be seen from U.S. Pat. No. 3,560,079, to increase the thickness of the doublet, (third lens group consisting of the fourth and fifth lens elements), to thereby promote the divergence of the light ray pencil for the purpose of increasing the back focal distance of the overall lens system. Additionally, it has been recognized that to achieve a satisfactory correction of coma and astigmatism, there must be provided a sufficiently broad air space (accommodating the diaphragm) between the second lens group doublet and the third lens group doublet. A Gauss type lens system of this design can be seen in the U.S. Pat. No. 3,634,001. The teachings of these United States Patents, however, do not benefit an optical designer who wishes to reduce the overall length of the lens system to achieve a compact design. The lens design suggested in these references would, by virtue of just the marginal thicknesses of the individual lens groups that would be required for commercial manufacturing and processing, increase the overall length of the lens system thus making it extremely difficult to accomplish the highly desired compactness required.
Generally, a five group, seven lens element Gauss type lens system having a back focal distance greater than 70 percent of its focal length, a normal field angle (2.omega.) of about 46 degrees and an F number of about 1.4 must have an overall length in excess of 95 percent of the focal length of the lens system. It is not believed that any five group Gauss type lens system heretofore previously known, has an overall length shorter than 90 percent of the focal length of the lens system.
Any Gauss type lens system heretofore known by the present inventor having an overall length less than 90 percent of its focal length is of the six group, seven lens element type in which the normal second group (doublet) is split so that the second and third lens elements are separated into two individual lens elements with an air lens provided therebetween. The air lens being utilized to correct the coma flare. While this design is effective in correcting coma flare, it is difficult to reduce the Petzval sum to a sufficient satisfactory extent with a short overall length design. The resulting lens system suffers in that the flatness of the image plane is not maintained and the correction of lateral chromatic aberration is made difficult. From a practical viewpoint, the manufacturing tolerances, particularly as effected by an eccentric error sensitivity, is decreased by the separated second and third lens elements. Accordingly, the commercial product will have a greater variance in its performance between individual manufactured lens systems than would be the case with the more conventional five group, seven element lens system having the second and third element cemented together as a doublet.
Finally, there is still a demand in the prior art for an improved compact Gauss type lens system that can be relatively easily manufactured for commercial sale.